The present invention relates to an R--T--B-based, permanent magnet with improved adhesion of a corrosion-resistant film layer to a magnet body as well as improved resistance to the deterioration of magnetic properties by working, electroplating, etc. and to their change with time, a permanent magnet-type motor and a permanent magnet-type actuator comprising such an R--T--B-based, permanent magnet.
Accompanied by recent market trend of miniaturization and weight reduction of electronic equipment and precision instrument, rare earth magnets have found wider applications in many fields in place of conventional Alnico magnets and ferrite magnets. Among the rare earth magnets, R--T--B-based, permanent magnets have particularly earned increasing needs because of high energy product, and higher energy product and coercivity tend to be required. However, since the R--T--B-based, permanent magnets are based on rare earth elements and iron, they are easily oxidized in the air to form stable oxides. Accordingly, if the R--T--B-based, permanent magnets without corrosion-resistant film layers are assembled into magnetic circuits of electronic equipment, etc., oxidation takes place on magnet surfaces and proceeds into the inside of the magnets. As a result, their magnetic properties are deteriorated, leading to reduction in performance of electronic equipment, etc., and oxides peel off from the magnet surfaces, contaminating adjacent elements with magnetic materials. For these reasons, various surface treatment methods have been proposed to prevent the oxidation of the R--T--B-based, permanent magnets.
For instance, there have been proposed resin coating methods by spraying or electrodeposition, gas deposition methods such as vacuum vapor deposition, ion sputtering, ion plating, etc., and electro- or electroless plating methods of metals or alloys of Cr, Ni, etc. For the R--T--B-based, permanent magnets which may be used at a temperature of 100.degree. C. or higher, metal plating, etc. is more utilized than coatings of resins having low glass transition temperatures from the aspect of cost and reliability.
However, if permanent magnets provided with corrosion-resistant film layers by electro- or electroless plating methods are used for permanent magnet-type motors or actuators which may be used at such an elevated temperature as 120.degree. C. or higher, deterioration of magnetic properties takes place in the permanent magnets, resulting in the reduction of performance of the permanent magnet-type motors or actuators.
In addition, because degreasing, activation, etc. with alkalis or acids are carried out as pretreatments in the electro- or electroless plating methods, grain boundary phases generating coercivity are dissolved away from the magnet surfaces in the course of the pretreatments, resulting in the formation of layers of low magnetic properties between main phases and corrosion-resistant film layers on the magnet surfaces, thereby lowering the magnetic properties of the permanent magnets. Particularly in the case of thin permanent magnets, the magnetic properties deteriorate drastically.
The formation of a low-magnetic properties phase is schematically shown in FIG. 1. In a sintered R--T--B magnet coated with a corrosion-resistant film layer 1, there are grain boundaries 3 between crystal grains 2 composed of a main phase of R.sub.2 Fe.sub.14 B, with a rare earth element-rich phase 4 and a boron-rich phase 5 therebetween. The rare earth element-rich phase 4 in contact with the corrosion-resistant film layer 1 is partly dissolved away, leaving layers 8 having low-magnetic properties. Some crystal grains 2 of the main phase may have cracks 7 near the low-magnetic properties areas 8.
To assemble the R--T--B-based, permanent magnets into electronic equipment, it is necessary to cut or grind totally or partially their surfaces before the formation of corrosion-resistant film layers. In this case, too, the magnet surfaces are disordered or plastically deformed to form damaged layers which may be referred to as "damaged layers," resulting in the reduction of magnetic properties. There is a disadvantage that corrosion-resistant film layers formed on the damaged layers are highly likely to peel off from the damaged layers. This is schematically shown in FIG. 2. There is a damaged layer 6 between the crystal grains 2 of the main phase and the corrosion-resistant film layer 1. It is also noted that crystal grains in the damaged layer 6 are severely cracked as shown by the reference numeral 7. The same reference numerals denote the same phases in FIGS. 1 and 2.